The only essential function of TFIIIA in yeast is the transcription of 5S rRNA genes.

We have developed a system to transcribe the yeast 5S rRNA gene in the absence of the transcription factor TFIIIA. A long transcript was synthesized both in vitro and in vivo from a hybrid gene in which the tRNA-like promoter sequence of the RPR1 gene was fused to the yeast 5S RNA gene. No internal initiation directed by the endogenous 5S rDNA promoter or any processing of the hybrid transcript was observed in vitro. Yeast cells devoid of transcription factor TFIIIA, which, therefore, could not synthesize any 5S rRNA from the endogenous chromosomal copies of 5S rDNA, could survive if they carried the hybrid RPR1-5S construct on a multicopy plasmid. In this case, the only source of 5S rRNA was the precursor RPR1-5S transcript that gave rise to two RNA species slightly larger than wild-type 5S rRNA. This establishes that the only essential function of TFIIIA is to promote the synthesis of 5S rRNA. However, cells devoid of TFIIIA and surviving with these two RNAs grew more slowly at 30 degrees C compared with wild-type cells and were thermosensitive at 37 degrees C.